Field of the Invention
This invention relates to internal combustion engines and more specifically to devices and methods for altering the acoustic signature of such engines.
Deception plays an important role in creating an advantage over opposing military forces. Even ancient military strategists were aware that deception is an integral component of war. A well known component of military deception includes a wide variety of camouflage techniques. The term "camouflage" originated from the French word "camoufler" meaning to disguise or play a trick. The "trick" is to fool the enemy into believing, seeing, or detecting that which you desire to be known. Some camouflage techniques used in the past include: deceive the enemy into believing that he sees no enemy forces or equipment; use fake tanks or aircraft mock-ups to create the illusion that more forces are present than actually exist or to create fake targets for enemy fire; and make identification of military equipment likely to be artillery targets more difficult by use of specially designed paint schemes intended to mask the presence of the target or make identification of specific portions of the target more difficult.
A major concern of military operations in the field is the acoustic detection of their fighting vehicles. Engine exhaust noise is the dominant source of noise under idle conditions. Tread noise dominates when vehicles (e.g. tanks) are in motion. The low frequency exhaust pulses of an idling engine are particularly easy to recognize with a simple spectrum analyzer and a microphone. Alterations of military vehicle acoustic signatures is analogous to stealth technology. The primary objective of both technologies is to evade enemy detection by altering or suppressing the detected radar or acoustic signature of a military fighting craft.
The periodic nature of engine exhaust noise at idle results in an audio frequency spectrum dominated by the firing frequency harmonic. All internal combustion engines, particularly Otto and Diesel cycle engines, will have a characteristic periodic exhaust noise which includes the firing frequency harmonic and higher order harmonics resulting from a number of engine parameters. Design parameters such as exhaust tubing diameter, cylinder dimensions and displacement, exhaust valve design, muffler dynamics and others influence the characteristic exhaust noise emanating from a vehicle. As is well known in the art, for every two revolutions of the crankshaft of an Otto or Diesel cycle engine, a firing cycle is completed. Thus, an engine idling at 480 RPM will repeat a particular firing pattern and produce a repeatable "noise signature" four times a second.
A device and method for altering the acoustic signature of an internal combustion is shown for example in copending application Ser. No. 489,528, by P. Hayes, T. Reinhart and T. Shaw filed concurrently herewith, titled "Device and Method for Altering the Acoustic Signature of an Internal Combustion Engine", the disclosure of which is hereby incorporated by reference. However, the cylinder cutout techniques of the related application can be costly to implement on existing engines.
A periodic exhaust pulse from an Otto or Diesel engine is particularly recognizable with a simple spectrum analyzer and a microphone. A device and method for altering the acoustic noise signature at idle of new and existing engines is needed to prevent acoustic identification of military vehicles under military conflict conditions.